In recent years a great number of genes has been characterized that are involved in different aspects of plant development and/or in the establishment of cell patterns underlying the different plant tissues and organs. Identification of such genes is usually based on isolation of mutagenized Arabidopsis thaliana plants exhibiting extreme or less extreme phenotypic aberrations in organs such as flowers, leaves or roots or exhibiting earlier defects, e.g. during embyro development. Without being exhaustive, genes involved in early establishment of flower meristem and organ identity include LEAFY (LFY), APETALA1 (AP1) and APETALA2 (AP2). Genes involved in later steps of flower organ identity patterning include APETALA3 (AP3), PISTILLATA (PI) and AGAMOUS (AG). Genes controlling the number of flower organs include CLAVATA1 (CLV1), CLAVATA3 (CLV3), ETTIN (ETT), PERIANTH (PAN) and TOUSLED (TSL). TOUSLED also regulates flower organ size. Flower meristem and organ identification further relies on the UNUSUAL FLOWER ORGAN (UFO) gene and flowering time is regulated by e.g. the CONSTANS (CO) and the LUMINIDEPENDENS (LD) genes. The inflorescence meristem can be held in an indeterminate state by e.g. the TERMINAL FLOWER 1 (TFL) gene. Many of the cited flower development genes encode transcription factors of e.g. the MADS-box class (e.g. AG), the ARF-class (e.g. ETT); or leucine-rich repeat-type receptor protein kinases (e.g. CLV1 and CLV2); or protein kinases (e.g. TSL) (Nemhauser et al. 1998, Sessions et al. 1997 and references cited in both; Aukerman et al. 1999, Pnueli et al. 1998). A naked, pin-formed inflorescence is formed in A. thaliana plants mutated in the PIN-FORMED1 (PIN1) gene (Palme and Gälweiler 1999).
Development of the shoot apical meristem (which is the source of leaf and flower primordia) relies on e.g. WUSCHEL (WUS) whereas its maintenance depends on e.g. the KNOTTED-like homeobox transcription factor gene SHOOT MERISTEMLESS (STM) and, during embryonic development, on the ZWILLE (ZLL) gene. Meristem size is regulated in early development by e.g. the PRIMORDIA TIMING (PT) gene and in later stages by e.g. CLV1. The rate of leaf formation is increased in clavata mutants. separation of organs emanating from the shoot apical meristem and separation of organs from each other relies on at least the CUP-SHAPED COTYLEDON (CUC1 and CUC2) genes and on AINTEGUMENTA (ANT). Initiation of lateral organ formation from the shoot apical meristem requires the MGOUN (MGO) genes. Leaf development is controlled by a number of genes including ARGONAUTE1 (AGO1), PHABULOSA (PHB) and PHANTASTICA (PHAN). Trichome formation from leaf epidermal cells involves e.g. GLABROUS1 (GL1), GLABRA2 (GL2), TRANSPARENT TESTA GLABRA (TTG), TRIPTYCHON (TRY) and ZWICHEL (ZWI) genes. Stomata patterning relies on e.g. GL2 and TTG (Benfey et al. 1999, Bowman and Eshed 2000, Doerner 1999, Langdale 1998, Lenhard and Laux 1999, McSteen and Hake 1998 and references cited in all).
The fate of root epidermal cells is controlled by genes such as GL2, TTG and CAPRICE (CPC). GL2 and TTG repress root hair formation whereas CPC, a MYB-type transcription factor, is a positive regulator of the root hair cell fate.
Establishment of the root cortex and root endodermis from the ground tissue involves the genes SCARECROW (SCR) encoding a putative transcriptional regulator and the related SHORT-ROOT (SHR). Both SCR and SHR might also stabilize endodermal cell identity. The MONOPTEROS (MP) gene is required for root and hypocotyl initiation. Depending on the strength of the mp mutant allele either the root or the root and the hypocotyl are lacking. (Benfey 1999, Helariutta et al. 2000, Scheres and Berleth 1998 and references cited therein).
A gene locus identified as HOBBIT (HBT), was shown to be involved in root meristem formation (Willemsen et al. 1998). Strong hbt mutant alleles result in impaired root meristem activity. Other defects in hbt mutant seedling roots are linked to lack of establishment of columella and lateral root cap cell identities. The hbt mutant phenotypes can be traced back to early defects in the development of the embryonal hypophyseal cell region. The ectopic formation of lateral root primordia and lateral roots in hbt mutant seedlings has also been described (Willemsen et al. 1998). According to the latter article it is described that the auxin production or the auxin perception are not generally defective in hbt mutants, however, the HBT gene function remains to be elucidated.
One of the problems thus underlying the present invention is to provide the isolated HBT gene and its functions together with particularly useful applications of said gene in agriculture, horticulture, and plant cell and tissue culture. A solution is achieved by providing the embodiments characterized in the claims.